1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device where display drive circuits for outputting display signals are disposed on a substrate of a display panel, and a display control circuit.
2. Description of the Related Art
As image display devices for personal computers and various other monitors, the use of liquid crystal devices is spreading remarkably. A liquid crystal display device typically has a liquid crystal display panel and a back light unit disposed on the rear face of the panel. The liquid crystal display panel displays images by controlling the transmission of light. One type of known liquid crystal device is the COG (Chip On Glass) type liquid crystal display device. In a COG type liquid crystal display device, a plurality of source driver ICs and/or a plurality of gate driver ICs are mounted on the glass substrate of the liquid crystal display panel. This can greatly contribute to reducing manufacturing cost.
Typically in a conventional liquid crystal display device, a timing controller and each source driver IC are connected by a separate line via the FPC (Flexible Printed Circuit). From the timing controller to each source driver IC, display signals and control signals are transmitted via each line. However, forming a line for each source driver IC makes the overall line length long and causes a cross-talk problem between lines. Thus, a method of cascade-connecting a plurality of source driver ICs with display signal transmission lines has been proposed.
The source driver ICs mounted on the glass substrate are cascade-connected for the transmission of display signals and control signals. Display signals and control signals outputted from the timing controller are inputted to the source driver IC in the first stage, which is disposed at the very edge of the substrate. When the latch processing of the display signals by the source driver IC in the first stage is over, the display signals are transmitted to the source driver IC in the next stage via the line on the substrate. The source driver IC in the second stage executes latch processing of the display signals according to the control signals, just as the source driver IC in the first stage. Hereafter, the source driver ICs in the subsequent stages repeat similar processing.
In the liquid crystal display device having cascade-connected COG type source driver ICs, a technique of decreasing the number of inputs of drivers and implementing COG & WOA (Line On Array) for cost reduction has been proposed (e.g. see Japanese Unexamined Patent Application Publication No. 2001-174843). In the liquid crystal display device, the source driver ICs, to which video signals inputted via video I/F are distributed, are cascade-connected and lines to each source driver IC are minimized to implement COG & WOA. In other words, this liquid crystal display device has liquid crystal cells that forms an image display area on the substrate and a source driver for applying a voltage to the liquid crystal cells based on the video signals inputted via the video I/F, and the source driver has a plurality of source driver ICs mounted on the same substrate as the liquid crystal cells and are cascade-connected by signal lines.
A typical source driver IC has a scan direction switching function. This function is used for ensuring flexibility in mounting the source driver IC and a correct display in a rotatable liquid crystal display device, which is used for a digital video and so on. For example, when a bare chip source driver IC is mounted on a TCP (Tape Carrier Package) in a TAB (Tape Automated Bonding) system, the chip is mounted on the rear face side or front face side of the TCP. Use of the scan direction switching function allows ICs with the same structure to be used for both the TCP and the COG of the rear face or front face packaging mode. It also allows the ICs with the same structure to be mounting on the top edge or the bottom edge of the substrate when the packaging mode is the same.
When the source driver ICs are cascade-connected for transmitting display signals, a conventional source driver IC must have a bi-directional buffer to switch the scan direction. The line for transmitting display signals is connected from the timing controller to the source driver IC at one end and to the source driver IC at the other end. To scan in a forward direction, the display signal is inputted to the source driver IC at the left end and transmitted to the source driver IC in the subsequent stage via the cascade-connection lines, for example. To scan in the backward direction, the display signal is inputted to the source driver IC at the right end and is transmitted to the source driver IC in the subsequent stage, in a direction opposite from the forward scan direction, via the cascade-connection lines. The transmission direction of each source driver IC is controlled by control signals.
In this way, having the bi-directional buffer, the input capacity of the source driver increases. If the capacity increases, the signal waveform is rounded, and the frequency with which the source driver IC normally operates drops. Further, the timing controller must have a display signal output terminal for each of the forward and backward scanning, which increases the number of terminals.